To win the job, Johnson will have to respond to Jim Harbaugh’s challenges and demands, his relentless prodding and probing, just as the quarterback did when they first met.

The coach has been challenging Johnson since their first conversation, when Harbaugh made a recruiting visit to Oakland Tech High.

“We didn’t know each other from a can of paint,” Johnson, now 26, recalled Wednesday at the team’s headquarters, where rookies and a handful of veterans practiced. “But we could talk football for hours. And he just kept pushing me, pushing me, trying to get a feel for my knowledge of the game.

“He brought out some pencils and had me drawing X’s and O’s from the day we met.”

Johnson followed Harbaugh to the University San Diego, and the marriage resulted in a slew of passing records — love at first flight. Johnson left USD as the school’s all-time leader in touchdown passes (113) and passing yards (9,699) and also ran for 1,864 yards and 19 TDs.

Reunited with Harbaugh after spending his first four NFL seasons with the Tampa Bay Buccaneers, Johnson will be battling for the spot behind starter Alex Smith when full training camp practices start Friday.

Already, he’s getting a daily reminder of that long-ago meeting.

“Coach Harbaugh doesn’t want you in that comfort zone,” Johnson said. “He always told me, ‘The reason I make it so hard for you in practice is so that the games are easy.’ “

Statistics suggest that Johnson (6-foot-3, 205 pounds) never made much progress in Tampa Bay. In four seasons coming mostly off the bench, he managed just five touchdown passes against 10 interceptions. He also fumbled eight times (though he lost only two).

On Wednesday, however, Johnson insisted that he was getting better behind the scenes, despite what the numbers say. He said he approached practice with the mentality he learned from Harbaugh. “The moment you start regressing, that’s when they try to replace you,” Johnson said.

Veteran receiver Randy Moss strolled into the locker room Wednesday as he reported for duty a day before required. Moss, 35, impressed coaches and teammates with his dedication during offseason OTAs, and his early arrival fueled hopes that the 49ers will be getting a fully motivated player in 2012.

Moss last played in 2010, when he amassed 393 yards and five touchdowns for three teams.

Jason Slowey, who played left tackle at Western Oregon, is learning all three interior offensive line positions. The 6-foot-4, 305-pounder is also making the adjustment from Division II competition to the NFL. “It’s a transition that hasn’t been as bad as everyone thought it would be — including me,” he said.

Offensive lineman Joe Looney, a fourth-round pick out of Wake Forest, put no timetable on his return from foot surgery but said he’s making progress. “I’m doing some running and whatnot,” he said.

A transit village with apartments, retailers, restaurants and a hotel is rising in Milpitas next to The Great Mall, close to light rail and the under-construction BART station. It’s one of several Silicon Valley projects sprouting up near transit.